Vehicle occupant restraint systems having an actuatable restraining device are well known in the art. One particular type of actuatable restraining device includes an inflatable air bag mounted to inflate within the occupant compartment of the vehicle. The air bag has an associated, electrically actuatable ignitor, referred to as a squib.
Such occupant restraint systems further include a crash sensor for sensing the occurrence of a vehicle crash condition and for providing an electrical signal indicative of the crash condition. When the crash sensor indicates that the vehicle is in a crash condition, an electric current of sufficient magnitude and duration is passed through the squib for the purpose of igniting the squib. The squib, when ignited, ignites a combustible gas generating composition and/or pierces a vessel of pressurized gas operatively coupled to the air bag, which results in inflation of the air bag.
Pressurized gas vessels for use in occupant restraint systems are pressurized to approximately 2000-3000 PSI. Proper inflation of the air bag upon the occurrence of a vehicle crash condition is dependent on the pressurized vessel being at the proper pressure. Since the pressurized vessel will be installed in the vehicle at the time of manufacture and several years may pass prior to that vehicle being involved in a crash condition, even a small leak in the vessel may leave the vessel with insufficient pressure to inflate the air bag properly during its deployment. It is desirable to provide a diagnostic test arrangement that monitors the fluid pressure in the vessel and warns the vehicle operator when the pressure in the vessel falls below a predetermined minimum value necessary for proper inflation of the air bag.
A pressurized vessel of an air bag restraint system has a normally sealed air bag opening which is pierced upon firing of the squib and through which fluid flows to the air bag. Several different methods and apparatus have been proposed to measure the pressure within the air bag pressure vessel and to warn the vehicle operator if the pressure falls below a predetermined value. These arrangements typically require that a pressure sensing device have access to the interior of the vessel through an associated test opening in the vessel. The test opening in the vessel is separate from the pierceable sealed air bag opening. If a pressurized vessel leaks, the location of such a leak is most probably at a sealed opening in the vessel. Such leaks may develop simply due to a failure of the seal. Therefore, the more openings that are present in the vessel, the greater the probability that a leak will develop. It is desirable to provide a pressure sensing method and apparatus that does not require an associated opening in the vessel, thereby avoiding an additional potential leak path of pressurized gas from the vessel.